Cell mediated hyperacute rejection (CMHAR) is the result of an as yet poorly understood immune response, the prime model being the white graft reaction of mice. The reaction is characterized by T cell mediation, a very brief sensitization period, a rapid response to challenge antigen, and a duration period of a week or less. Another example - or analog to this murine white graft reaction - appears to be 51 Cr release from suitable targets exposed to lymphoid cells from mice sensitized to give rise to a white graft reaction: similar kinetics and effector cells. White graft reactions in other species - e.g. man, rat, monkey - are characterized by serum mediation and different kinetics. Much of the present investigation is aimed at further characterization of the effector cells, by transfer experiments and several standard in vitro assays (e.g. 51 Cr release, MLC), and also the effector pathways, - e.g. whether transferred sensitized cells are the actual killers, or whether they merely recruit a host mechanism. Finally, the events at the host graft interface will be subjected to a more detail scrutiny, both from an ultra structural and immunologic (immunofluorescent) point of view.